Advances in thin film technology have enabled the development of sophisticated integrated circuits. This advanced semiconductor technology has also been leveraged to create MEMS (Micro Electro Mechanical System) structures. MEMS structures are typically capable of motion or applying force. Many different varieties of MEMS devices have been created, including microsensors, microgears, micromotors, and other microengineered devices. These MEMS devices can be employed in a variety of applications including hydraulic applications in which pumps and valves are used and optical applications that include MEMS light valves and shutters. Currently, MEMS devices are being developed for a wide variety of applications because they provide the advantages of low cost, high reliability and extremely small size.
Design freedom afforded to engineers of MEMS devices has led to the development of various techniques and structures for providing the force necessary to cause the desired motion within microstructures. For example, microcantilevers have been used to apply rotational mechanical force to rotate micromachined springs and gears. Electromagnetic fields have been used to drive micromotors. Piezoelectric forces have also successfully been used to controllably move micromachined structures. Controlled thermal expansion of actuators or other MEMS components has been used to create forces for driving microdevices. One such device is found in U.S. Pat. No. 5,475,318 entitled "Microprobe" issued Dec. 12, 1995 in the name of inventors Marcus et al., which leverages thermal expansion to move a microdevice. A micro cantilever is constructed from materials having different thermal coefficients of expansion. When heated, the bimorph layers arch differently, causing the micro cantilever to move accordingly. A similar mechanism is used to activate a micromachined thermal switch as described in U.S. Pat. No. 5,463,233 entitled "Micromachined Thermal Switch" issued Oct. 31, 1995 in the name of inventor Norling.
In addition, U.S. Pat. No. 5,909,078 entitled "Thermal Arched Beam Microelectromechanical Actuators" which issued Jun. 1, 1999 in the name of inventors Wood, et al., describes thermal actuators having a pair of arched beams extending between a pair of supports disposed on a microelectronic substrate. By passing current through the arched beams, the arched beams will expand so as to further arch. The thermal actuator of the Wood patent can also include an actuator member that connects a plurality of arched beams and serves to push against a workpiece.
The need exists to develop MEMS actuated variable optical attenuators that will benefit from the low cost fabrication, high reliability and size advantage that are characteristic of similar MEMS structures. Of particular importance in optical attenuation is the need to fabricate devices that are variable over a full optical power range and benefit from low insertion loss. By providing a device capable of attenuating optical power across a much larger dynamic power range, it would be possible to attenuate beams that have a much wider optical beam and/or an unfocussed beam at the point of attenuation. Additionally, a MEMS actuated variable optical attenuator would provide for finer and more precise control over the optical attenuation allowing for the transmitted optical power to be dynamically altered as required by the specific application. It is also desirable to devise a MEMS actuated variable optical attenuator that would benefit from less power consumption. To date, however, MEMS activated variable optical attenuators are not available, at least not commercially, even though such MEMS variable optical attenuators will likely be instrumental in future light wave communication systems and optoelectronic systems.